Microwave combiner and distributer for quantum signals using frequency-division multiplexing

ABSTRACT

A technique relates to a superconducting microwave combiner. A first filter through a last filter connects to a first input through a last input, respectively. The first filter through the last filter each has a first passband through a last passband, respectively, such that the first passband through the last passband are each different. A common output is connected to the first input through the last input via the first filter through the last filter.

DOMESTIC PRIORITY

This application is a continuation of U.S. application Ser. No.16/153,870, titled “MICROWAVE COMBINER AND DISTRIBUTER FOR QUANTUMSIGNALS USING FREQUENCY-DIVISION MULTIPLEXING” filed Oct. 8, 2018, whichis a continuation of U.S. application Ser. No. 15/275,511, titled“MICROWAVE COMBINER AND DISTRIBUTER FOR QUANTUM SIGNALS USINGFREQUENCY-DIVISION MULTIPLEXING” filed Sep. 26, 2016, which is now U.S.Pat. No. 10,164,724, the contents of which are incorporated by referenceherein in their entireties.

BACKGROUND

The present invention relates to superconducting electronic devices, andmore specifically, a microwave combiner and distributer for quantumsignals using frequency-division multiplexing.

In telecommunications, frequency-division multiplexing is a technique bywhich the total bandwidth available in a communication medium is dividedinto a series of non-overlapping frequency sub-bands, each of which isused to carry a separate signal. This allows a single transmissionmedium such as the airways, a cable, or an optical fiber to be shared bymultiple independent signals.

In physics and computer science, quantum information is information thatis held in the state of a quantum system. Quantum information is thebasic entity of study in quantum information theory, and can bemanipulated using engineering techniques known as quantum informationprocessing. Much like classical information can be processed withdigital computers, transmitted from place to place, manipulated withalgorithms, and analyzed with the mathematics of computer science,analogous concepts apply to quantum information. Quantum systems such assuperconducting qubits are very sensitive to electromagnetic noise,particularly in the microwave and infrared domains.

SUMMARY

According to one or more embodiments, a superconducting microwavecombiner is provided. The superconducting microwave combiner includes afirst filter through a last filter. The first filter through the lastfilter connects to a first input through a last input, respectively. Thefirst filter through the last filter each has a first passband through alast passband, respectively, such that the first passband through thelast passband are each different. Also, the superconducting microwavecombiner includes a common output connected to the first input throughthe last input via the first filter through the last filter.

According to one or more embodiments, a method of configuring asuperconducting microwave combiner is provided. The method includesproviding a first filter through a last filter, where the first filterthrough the last filter connects to a first input through a last input,respectively. The first filter through the last filter each has a firstpassband through a last passband, respectively, such that the firstpassband through the last passband are each different. Also, the methodincludes providing a common output connected to the first input throughthe last input via the first filter through the last filter.

According to one or more embodiments, a superconducting microwavedistributer is provided. The superconducting microwave distributerincludes a first filter through a last filter, where the first filterthrough the last filter connects to a first output through a lastoutput, respectively. The first filter through the last filter has afirst passband through a last passband, respectively, such that thefirst passband through the last passband are each different. Also, thesuperconducting microwave distributer includes a common input connectedto the first output through the last output via the first filter throughthe last filter.

According to one or more embodiments, a method of configuring asuperconducting microwave distributer is provided. The method includesproviding a first filter through a last filter, where the first filterthrough the last filter connect to a first output through a last output,respectively, wherein the first filter through the last filter has afirst passband through a last passband, respectively, such that thefirst passband through the last passband are each different. The methodincludes providing a common input connected to the first output throughthe last output via the first filter through the last filter.

According to one or more embodiments, a superconducting system isprovided. The superconducting system includes a first quantum systemthrough a last quantum system, and a first filter through a last filter.The first filter through the last filter connects to the first quantumsystem through the last quantum system, respectively. The first filterthrough the last filter has a first passband through a last passband,respectively, such that the first passband through the last passband areeach different. The superconducting system includes a common outputconnected to the first quantum system through the last quantum systemvia the first filter through the last filter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a device depicting a microwave combiner forquantum signals according to one or more embodiments.

FIG. 2 is a schematic of the device depicting a microwave distributerfor quantum signals according to one or more embodiments.

FIG. 3 is a system depicting the device utilized in a quantum systemapplication according to one or more embodiments.

FIG. 4 illustrates devices as a cascading tree of power combinersaccording to one or more embodiments.

FIG. 5 is a schematic of the device depicting a microwave combiner forquantum signals according to one or more embodiments.

FIG. 6 is a flow chart of a method of configuring a superconductingmicrowave combiner device according to one or more embodiments.

FIG. 7 is a flow chart of a method of configuring a superconductingmicrowave distributer device according to one or more embodiments.

DETAILED DESCRIPTION

Various embodiments are described herein with reference to the relateddrawings. Alternative embodiments can be devised without departing fromthe scope of this document. It is noted that various connections andpositional relationships (e.g., over, below, adjacent, etc.) are setforth between elements in the following description and in the drawings.These connections and/or positional relationships, unless specifiedotherwise, can be direct or indirect, and are not intended to belimiting in this respect. Accordingly, a coupling of entities can referto either a direct or an indirect coupling, and a positionalrelationship between entities can be a direct or indirect positionalrelationship. As an example of an indirect positional relationship,references to forming layer “A” over layer “B” include situations inwhich one or more intermediate layers (e.g., layer “C”) is between layer“A” and layer “B” as long as the relevant characteristics andfunctionalities of layer “A” and layer “B” are not substantially changedby the intermediate layer(s).

Employing microwave power combiners in order to combine quantum signalsin the microwave domain comes at the expense of impedance mismatchesbetween the ports (which causes reflections), poor isolation between(certain) ports, and/or loss of quantum information due to attenuationof the quantum signal. The loss of quantum information can be either dueto internal loss of the power combiner or leakage to other ports. Thisloss of quantum information can result in a significant decrease in thesignal-to-noise ratio of the measurement.

Furthermore, in a scalable quantum processor architecture based onsuperconducting qubits, it is important to minimize the number of inputand output lines that go into a dilution fridge. One way to achieve thison the output side is, for example, by combining multiple readoutsignals of multiple qubits on the same output line using hybrids orcommercial power combiners. Subsequently, a broadband quantum-limitedamplifier can be utilized to amplify the multiple readout signals beforethey propagate up the output chain. However, using such hybrids or powercombiners attenuate the quantum signals before the amplifier and resultin significant loss of quantum information, thus limiting the efficiencyof the quantum measurement.

Another possibility is to add the power combiners following multiplequantum-limited amplifiers which amplify each individual signal.However, the downside of this scheme is the addition of bulky hardware(multiple amplifiers) to the dilution fridge which limits scalability ofsuch as scheme. A dilution refrigerator is a cryogenic device thatprovides continuous cooling to temperatures as low as 7 millikelvin(mK), with no moving parts in the low-temperature region.

Below are a few examples of power combiners or dividers.

The following are characteristics of a T-Junction power combiner: 1)3-port device with one input port and two output ports, 2) lossless, 3)reciprocal, 4) no isolation between the output ports, and 5) matchedonly to the input.

Characteristics of a resistive divider include the following: 1) 3-portdevice, 2) reciprocal, 3) can be matched at all ports (no reflection),4) lossy, and 5) no isolation between the output ports.

Characteristics of hybrids (90° and 180° hybrids) include thefollowing: 1) 4-port devices with two input ports and two output ports,2) reciprocal, 3) can be matched at all ports (no reflection), 4) goodisolation between the two input ports, and between the two output ports.If the hybrids are used as a power combiner, the power is dividedequally between the two output ports such that half of the informationis lost.

The following are characteristics of a Wilkinson powerdivider/combiner: 1) 3-port device with one input and two output ports(can be generalized to N-way device), 2) matched at all ports (noreflection), 3) isolation between the two output ports, 4) lossy whencombining signals that are input on the output ports, because only halfof the signal power shows up at the input, while the other half isdissipated.

One or more embodiments address problems or issues related tomultiplexing the drive and readout of multiple qubit-resonator systemsusing a small number of input and output lines (thereby providingscalability), without causing loss of quantum information (lossless),and without allowing leakage of signals between differentqubit-resonator systems (isolation between input ports and/or isolationbetween output ports). Embodiments include one or more methods anddevices that separate the microwave signals (drive signals and readoutsignals) based on their frequency, thus allowing the methods and devicesto multiplex the readout and drive of multiple qubits withoutattenuating the microwave signals used in the drive/readout. Also, thedevices are provided with isolation between the different ports.

According to one or more embodiments, the device can be a multiporton-chip superconducting microwave combiner and/or distributer forquantum signals. The microwave combiner and distributer are lossless andtheir ports are matched to the input/output environment. Thesuperconducting microwave combiner and/or distributer can be used inscalable quantum processing architectures, such as for qubit drive andreadout.

In one or more embodiments, a microwave combiner and distributer areconfigured in the same device. The direction of the input signalsdetermines if the device is operating as a microwave combiner ordistributer.

FIG. 1 is a schematic of a device depicting a microwave combiner forquantum signals according to one or more embodiments. The device 100 isconfigured to utilize frequency-division multiplexing to allocatedifferent frequencies for different microwave signals onto a singleoutput transmission line.

The device 100 includes bandpass microwave filters generally referred toas bandpass filters 105. The different bandpass filters 105 are depictedas bandpass filters 105_1 through bandpass filters 105_N. Each bandpassfilter 105 has a different narrow passband through which microwavesignals having a frequency in the particular narrow passband aretransmitted (i.e., passed) and signals having a frequency outside of theparticular narrow passband are reflected (i.e., blocked). The bandpassfilter 105_1 has its own narrow passband with a bandwidth 1 (BW₁),bandpass filter 105_2 has its own narrow passband with a bandwidth 2(BW₂), and bandpass filter 105_N has its own narrow passband with abandwidth N (BW_(N)).

For example, bandpass filter 105_1 is configured with a passband(frequency band) that permits a microwave signal 150_1 having frequencyf₁ to pass (transmit) through but blocks (reflects) all other microwavesignals 150_2 through 150_N having frequencies f₂ through f_(N) whichare outside of the passband for bandpass filter 105_1. Similarly,bandpass filter 105_2 is configured with a passband (frequency band)that permits a microwave signal 150_2 having frequency f₂ to pass(transmit) through but blocks (reflects) all other microwave signals150_1, 150_3 through 150_N having frequencies f₁, f₃ through f_(N) whichare outside of the passband for bandpass filter 105_2. Analogously,bandpass filter 105_N is configured with a passband (frequency band)that permits a microwave signal 150_N having frequency f_(N) to pass(transmit) through but blocks (reflects) all other microwave signals150_1 through 150_N-1 having frequencies f₁ through f_(N-1) which areoutside of the passband for bandpass filter 105_N. The microwave signals150_1 through 150_N are generally referred to as microwave signals 150.When cavity-qubit quantum systems are operatively connected to thedevice 100, the microwave signals 150 can be at respective frequenciesf₁ through f_(N) designated to drive particular qubits or designated toreadout qubit (via readout resonators or cavities), as understood by oneskilled in the art.

As an example, one bandpass filter 105 can have a passband of 1 MHz,another bandpass filter 105 can have a passband of 10 MHz, yet anotherbandpass filter 105 can have a passband of 100 MHz, and so forth.

The device 100 includes ports 110 individually connected to respectivebandpass filters 105. Particularly, the different ports 110 aredesignated as port 1, port 2 through port N, where port N represents thelast of the ports 110. Similarly, N represents the last of thefrequencies, microwave signals 150, bandpass filters 105, quantumsystems 305 (discussed in FIG. 3), and so forth. In the device 100, port1 is connected to bandpass filter 105_1, port 2 is connected to bandpassfilter 105_2, and port N is connected to bandpass filter 105_N. Eachport_1 through port_N is connected to one end of its own bandpass filter105_1 through bandpass filter 105_N. The other end of the bandpassfilter 105_1 through bandpass filter 105_N is connected to a common port120 via a common node 115. The common node 115 can be a commonconnection point, a common transmission line, a common wire, etc., as amutual location for electrical connection. The common port 120 connectsto each bandpass filter 105_1 through bandpass filter 105_N, while theindividual ports 110 (ports 1-N) are connected (only) to theirrespective bandpass filter 105_1 through bandpass filter 105_N.

Because the bandpass filters 105_1 through 105_N only transmitrespective microwave signals 150_1 through 150_N in the respectivepassband, the device 100 is configured such that each bandpass filter105_1 through bandpass filter 105_N covers a different band (orsub-band) of frequencies, such that none of the passbands (of thebandpass filters 105) are overlapping. Accordingly, each port 1, port 2,through port N is isolated from one another because of being connectedto its respective bandpass filter 105_1 through 105_N, such that nomicrowave signal 150 through any one port 110 (whether entering orexiting) leaks into another port 110 via the common node 115. As sucheach port 110 is isolated from other ports 110 and is designed totransmit its own microwave signal 150 at a predefined frequency (orwithin a predefined frequency band), as a result of being connected toits own bandpass filter 105. As such, the bandpass filters 105_1 through105_N are responsible for providing the isolation among ports 110 (e.g.,port 1, port 2 through port N)

The respective ports 110, bandpass filters 105, common node 115, andcommon port 120 are connected to one another via transmission lines 30.The transmission line 30 can be a stripline, microstrip, etc. Themicrowave bandpass filters 105 are designed and implemented usinglossless or low loss lumped elements such as superconducting inductors,superconducting gap capacitors and/or plate capacitors, and passivesuperconducting elements. The superconducting elements includelumped-element inductors, gap capacitors, and/or plate capacitors (withlow loss dielectrics). Other possible implementations of the bandpassfilters include coupled-line filters, and/or capacitively-coupled seriesresonators.

The respective ports 110, bandpass filters 105, common node 115, commonport 120, and transmission lines 30 are made of superconductingmaterials. Examples of superconducting materials (at low temperatures,such as about 10-100 millikelvin (mK), or about 4 K) include niobium,aluminum, tantalum, etc.

In one implementation of the device 100 as a microwave combiner, acoaxial cable can connect to the external ends of the ports 110 and 120such that coaxial cables connected to ports 110 input microwave signals150_1 through 150_N at different frequencies f₁ through f_(N) whileanother coaxial cable connected to common port 120 outputs the combinedmicrowave signals 150_1 through 150N. In the microwave combiner, foreach microwave signal 150_1 through 150N at its respective frequency f₁through f_(N), none of the microwave signals 150 are transmitted backthrough any of the other (input) ports 110 (i.e., port isolation), butinstead each microwave signal 150_1 through 150_N passes through itsrespective bandpass filter 105_1 through 105_N, passes through thecommon node 115, and exits through the common port 120. As such, themicrowave combiner combines the microwave signals 150_1 through 150_Nand outputs them through the common port 120. The device 100 isconfigured with the frequency relation f₁<f₂< . . . <f_(N), where eachfrequency f₁, f₂, . . . f_(N) is the center frequency of the bandpassfilters 105_1 through 105_N, respectively. The device 100 is configuredsuch that it satisfies the inequality

$\frac{{BW}_{j} + {BW}_{i}}{2} < {{f_{j} - f_{i}}}$where i,j=1, 2, . . . N and j≠i. This inequality requires that thefrequency spacing between the center frequencies of each pair ofbandpass filters exceeds their average bandwidths. In other words, theinequality ensures that none of the bandpass filters have overlappingbandwidths (i.e., frequency range).

Each port 1 through port N with its respective transmission line 30 (andrespective bandpass filter 105_1 through 105_N) is considered adifferent/separate channel/input, and common port 120 is a commonchannel. Accordingly, when operating as a power combiner, multiple inputchannels of ports 1 through N are connected to the (single) commonchannel of common port 120. The device 100 is configured to bebidirectional. As noted herein, the same device 100 can be utilized as aboth a microwave power combiner and microwave signal distributer.

FIG. 2 is a schematic of the device 100 depicting a microwavedistributer for quantum signals according to one or more embodiments.The microwave distributer device 100 is configured to distribute themicrowave signals 150_1 through 150_N input on the common port 120 toindividual ports 1 through N, in which the microwave signals 150_1through 150_N are directed/distributed according to the passband of therespective bandpass filter 105_1 through 105_N.

In one implementation of the device 100 as the microwave distributer, acoaxial cable can connect to the external ends of the common port 120such that the coaxial cable connected to the common port 120 inputs themicrowave signals 150_1 through 150_N at different frequencies f₁through f_(N) while other coaxial cables connected to output ports 110output the individual microwave signals 150_1 through 150N. In themicrowave distributer, for each microwave signal 150_1 through 150N atits respective frequency f₁ through f_(N), only individual frequenciesf₁ through f_(N) are permitted to pass through the respective bandpassfilters 105_1 through 105_N having a passband covering the correspondingfrequency f₁ through f_(N), thus passing through individual port 1through port N. Because each of the bandpass filters 105_1 through 105_Nhas no overlapping passband, each microwave signal 150_1 through 150_Nhas its own frequency f₁ through f_(N) predefined to only pass throughone of the bandpass filters 105_1 through 105_N. The microwave signals150 at its own one of the frequencies f₁ through f_(N) are input throughthe common port 120, and each microwave signal 150_1 through 150_Npasses through the common node 115, is transmitted through itsrespective bandpass filter 105_1 through 105_N, and exits throughindividual ports 1-N according to frequency f₁ through f_(N). Each port1-N (only) outputs its own respective frequency f₁ through f_(N) becauseof the filtering by the respective bandpass filters 105_1 through 105_N.In other words, port 1 outputs microwave signal 150_1 at frequency f₁(via bandpass filter 105_1), while bandpass filter 105_1 blocksfrequencies f₂-f_(N). Port 2 outputs microwave signal 150_2 at frequencyf₂ (via bandpass filter 105_2), while bandpass filter 105_2 blocksfrequencies f₁, f₃-f_(N). Similarly, port N outputs microwave signal150_N at frequency f_(N) (via bandpass filter 105_N), while bandpassfilter 105_N blocks frequencies f₁-f_(N-1).

In FIG. 2, each port 1 through port N with its respective transmissionline 30 (and respective bandpass filter 105_1 through 105_N) isconsidered a different/separate channel/output, and common port 120 is acommon channel/input. Accordingly, when operating as a powerdistributer, multiple output channels of ports 1 through N are connectedto the (single) common (input) channel of common port 120.

As can be recognized in FIGS. 1 and 2, the device 100 is configured tobe operated as both a microwave signal distributer and combineraccording to whether the ports 110 or 120 receive input of the microwavesignals 150.

FIG. 3 is a system 300 depicting the device 100 utilized in a quantumsystem application according to one or more embodiments. FIG. 3 is anexample application of the device 100 depicting frequency-multiplexedreadout of qubits by having the microwave signals 150_1 through 150_Nwith frequencies f₁-f_(N) that match or nearly match the respectiveresonance frequencies of the cavities/resonators 1-N. It should beappreciated that the example can be equally applied to drive the qubitby instead having the microwave signals 150_1 through 150_N withfrequencies f₁-f_(N) that match or nearly match the respective resonancefrequencies of the qubits 1-N.

In the system 300, quantum systems 305_1 through 305_N are respectivelyconnected to the (input) ports 1 through port N. The quantum systems canbe generally referred to as quantum systems 305. The quantum system305_1 can be a cavity and qubit 1 operatively coupled together. Thequantum system 305_2 can be a cavity and qubit 2 operatively coupledtogether. Similarly, the quantum system 305_N can be a cavity and qubitN operatively coupled together. In the quantum system 305, the cavityand qubit can be capacitively connected, can be connected in atwo-dimensional cavity, and/or can be connected in a three-dimensionalcavity as understood by one skilled in the art. One type of qubit is asuperconducting qubit containing at least one Josephson junction, wherea Josephson junction is a nonlinear non-dissipative inductor formed oftwo superconducting metals (e.g., aluminum, niobium, etc.) sandwiching athin insulator such as, for example, aluminum oxide, niobium oxide, etc.

In one implementation, the system 300 can also include a widebandquantum-limited amplifier 350 connected to the (output) common port 120.The wideband quantum-limited amplifier 350 has a wide bandwidth designedto amplify all the microwave signals 150 having respective frequenciesf₁ through f_(N).

Each quantum system 305 is designed to resonate at its own resonancefrequency which is different for each quantum system 305. One skilled inthe art recognizes that the cavity in each quantum system 305 is oroperates as a resonator, such that the cavity resonates at its ownresonance frequency, typically called a readout resonator frequency.Particularly, the cavity in the quantum system 305_1 is configured toresonate at its resonance frequency, for example, which is frequency f₁.The cavity in the quantum system 305_2 is configured to resonate at itsresonance frequency which is frequency f₂. Similarly, the cavity in thequantum system 305_N is configured to resonate at its resonancefrequency which is frequency f_(N).

The quantum systems 305 are coupled to the device 100 via capacitors325, and the quantum systems 305 are coupled to the external environmentvia capacitors 320. The external environment can include microwavesignal generation equipment.

During frequency-multiplexed readout of the respective qubit in thequantum system 305_1 in the system 300, the microwave signal 150_1 atfrequency f₁ is at the resonance frequency for the cavity in the quantumsystem 305_1, and the microwave signal 150_1 is at the frequency f₁ totarget the both port 1 and the bandpass filter 105_1 (because thebandpass filter 105_1 is designed to pass frequency f₁). Duringfrequency-multiplexed readout of the respective qubit in the quantumsystem 305_2, the microwave signal 150_2 at frequency f₂ is at theresonance frequency for the cavity in the quantum system 305_2, and themicrowave signal 150_2 is at the frequency f₂ to target the both port 2and the bandpass filter 105_2 (because the bandpass filter 105_2 isdesigned to pass frequency f₂). During frequency-multiplexed readout ofthe respective qubit in the quantum system 305_N, the microwave signal150_N at frequency f_(N) is at the resonance frequency for the cavity inthe quantum system 305_N, and the microwave signal 150_N is at thefrequency f_(N) to target the both port N and the bandpass filter 105_N(because the bandpass filter 105_N is designed to pass frequency f_(N)).The microwave signals 150_1 through 150_N at the respective resonancefrequencies f₁ through f_(N) cause the quantum systems 305_1 through305_N to respectively resonate, and therefore, the microwave signals 150(at the respective resonance frequencies) cause the readout of therespective qubits coupled to their respective cavity (resonator). Assuch, the microwave signal 150_1 after interacting with the quantumsystem 305_1 (i.e., the qubit-resonator) is transmitted through port 1,to the bandpass filter 105_1, through the common port 120, and to thewideband quantum-limited amplifier 350. The microwave signal 150_2 afterinteracting with the quantum system 305_2 (i.e., the qubit-resonator) istransmitted through port 2, to the bandpass filter 105_2, through thecommon port 120, and to the wideband quantum-limited amplifier 350.Similarly, the microwave signal 150_N after interacting with the quantumsystem 305_N (i.e., the qubit-resonator) is transmitted through port N,to the bandpass filter 105_N, through the common port 120, and to thewideband quantum-limited amplifier 350. After interacting with therespective quantum system 305_1 through 305_N, each of the microwavesignals 150_1 through 150_N contains quantum information (e.g., thestate) of the respective qubits. Each of the microwave signals 150_1through 150_N are (simultaneously) amplified by the widebandquantum-limited amplifier 350.

The quantum signal is a microwave signal. It should be recognized thatthe microwave signal 150 can be bi-directionally transmitted in thedevice 100.

FIG. 4 illustrates devices 100 as a cascading tree of power combinersaccording to one or more embodiments. FIG. 4 is an example of scaling upthe devices 100. The devices 100 are configured to be fabricated on awafer, for example, as a chip. So as not to unnecessarily obscure theFIG. 4, some details of the devices 100 have been omitted for the sakeof clarity. It is understood that these details are included by analogyas discussed herein.

In this example, the tree of power combiners is depicted with 2 levels.In other implementations, there can be 3, 4, 5 . . . 10 or more levelsin the tree of power combiners. In FIG. 4, there can be M units of thedevices 100 in level 2, and the M units of devices 100 each have Ninputs in level 2. Having N inputs means that each device 100 in level 2has the corresponding number of N ports 110 connected to theirrespective one of the N bandpass filters 105. As discussed herein, eachof the N inputs has a single port 110 and single bandpass filter 105 ona one-to-one basis. In level 2, the devices 100 have bandpass filters105 in which each of the bandpass filters 105 has a different passband(i.e., a different frequency band) as discussed herein, such that thereis no overlap in their coverage of frequencies.

Each of the power combiner devices 100_1 through 100_M is configured tooutput microwave signals 150 on its respective center transmission line30_1 through 30_M. The designation of transmission line 30_1 through30_M is utilized to show that each one of the power combiner devices100_1 through 100_M has its own output transmission line 30, andaccordingly, the total number of center transmission lines 30 from thedevices 100 in level 2 is equal to M. In level 1, the device 100_Z has Mnumber of inputs. The output of each device 100_1 through 100_M isindividually connected to its own one of the M number of inputs of thedevice 100_Z, such that each of the center transmission lines 30_1through 30_M is one of the M inputs of the device 100_Z.

The device 100_Z is identical to the devices 100 discussed herein.However, the device 100_Z is structured such that each of the M inputshas its own connected bandpass filter 105 in level 1 with a passbandthat covers all of the passbands of the bandpass filters 105 in thelower level 2 per center transmission line 30. For example, in level 1of the tree 400, device 100_Z has a first input (of the M inputs) withport 1 so that its bandpass filter 105 in level 1 includes all of thepassbands of the bandpass filters in the device 100_1 in the level 2.Similarly, in level 1 of the tree 400, device 100_Z has a second input(of the M inputs) with port 2 so that its bandpass filter 105 in level 1includes all of the passbands of the bandpass filters in the device100_2 (not shown) in the level 2. Through the last input (of the Minputs) in level 1 of the tree 400, device 100_Z has a last input withport N so that its bandpass filter 105 in level 1 includes all of thepassbands of the bandpass filters in the device 100_M in the level 2.

In level 1, the device 100_Z is configured to receive the microwavesignal 150_1 through 150_Z on the M inputs and combine the microwavesignals 150_1 through 150_Z to be output on the center transmission line30_Z. Accordingly, the tree 400 of power devices is scaled up such thatthe level 1 device 100_Z outputs M×N microwave signals 150 whichcorrespond to M units of the devices 100 in level 2 each of the devices100 in level 2 having N inputs. The direction of the microwave signals150_1 through 150_Z shows the tree 400 operating as a scaled-up powercombiner. Analogously, the direction of the microwave signals 150_1through 150_Z can be switched to operate as a scaled-up signaldistributer.

FIG. 5 is a schematic of the device 100 depicting a microwave combinerfor quantum signals according to one or more embodiments. The device 100includes all the various features discussed herein. Further, the device100 includes additional features to ensure impedance matching for thepassing microwave signals (i.e., minimize reflections along the signalpath), and also enable the connection of multiple branches/lines to thecommon node 115.

In FIG. 5, impedance transformers 505_1 through 505_N are respectivelyadded between the respective ports 1 through N and their associatedbandpass filters 105_1 through 105_N. Also, the device 100 includes awideband impedance transformer 510 connected to the common node 115 andthe common port 120. The impedance transformers 505_1 through 505_N andimpedance transformer 510 are configured to provide impedance matching.On one end of the device 100, the impedance transformers 505_1 through505_N are structured to match (or nearly match) the input impedance Z₀of the ports 1-N and to match the associated bandpass filter 105_1through 105_N. Each of the impedance transformers 505_1 through 505_N isconfigured with a characteristic impedance Z=√{square root over(Z₀Z_(H))}, where Z₀ is the input impedance (as well as the outputimpedance), where Z_(H) is the high impedance of the bandpass filters105_1 through 105_N, and where Z is the average impedance of eachimpedance transformers 505_1 through 505_N. The average characteristicimpedance Z is the square root of the product of Z₀ and Z_(H). Onereason why transforming the impedance of the device ports Z₀ to highcharacteristic impedance Z_(H) in the region of the common node can beuseful, is because, in general, high impedance transmission lines, suchas a microstip or stripline, have narrow traces which in turn minimizethe physical size of the common node and allows more lines to be joinedtogether at that node. This is particularly relevant if the bandpassfilters are implemented as coupled-line filters and/orcapacitively-coupled resonators. If, however, all filters areimplemented using lumped-elements (with a very small footprint), suchimpedance transformations might be less of a concern.

In one implementation, the impedance transformers 505_1 through 505_Ncan be impedance matching transmission lines where one end (e.g., leftend) has a wide width matching the input impedance Z₀ and the oppositeend (e.g., right end) has a narrow width matching the high impedanceZ_(H) of the bandpass filters 105. Each of the impedance matchingtransformers 505_1 through 505_N has a length according to its ownrespective relationship λ₁/4, λ₂/4, . . . , λ_(N)/4, where λ₁ is thewavelength of the microwave signal 150_1, where λ₂ is wavelength of themicrowave signal 150_2, through λ_(N) which is the wavelength of themicrowave signal 150_N. These impedance transformers have in generalnarrow bandwidths.

In one implementation, the wideband impedance transformer 510 can be animpedance matching transmission line where one end (e.g., left end) hasa narrow width matching the high impedance Z_(H) of the bandpass filters105 (via common node 115) while the opposite end (e.g., right end) has awide width matching the output impedance Z₀. Such a wideband impedancetransformer can be implemented using tapered transmission lines, forexample, transmission lines whose widths are changed adiabatically onthe scale of the maximum signal wavelength. Other implementations oftapered lines known to one skilled in the art are possible as well, suchas the Exponential Taper or the Klopfenstein Taper. Also, it should benoted that the wideband requirement for this impedance transformerversus the other transformers 505, arises from the fact that thiswideband transformer needs to match the characteristic impedance for awideband of signal frequencies transmitted through it, in contrast tothe impedance transformers 505 which need only to match the impedancefor a narrow frequency range centered around the corresponding centerfrequency of the bandpass.

The impedance transformers 505_1 through 505_N and impedance transformer510 are made of superconducting material as discussed herein, such as,for example, niobium, aluminum, tantalum, etc.

The impedance designation Z₀ is the characteristic impedance at ports110 and 120 (which can be the input and output ports or vice versa). Forexample, the characteristic impedance Z₀ can be 50 ohms (Ω) at each port110 and 120 as recognized by one skilled in the art.

FIG. 6 is a flow chart of a method 600 of configuring a superconductingmicrowave combiner device 100 according to one or more embodiments.Reference can be made to FIGS. 1-5.

At block 605, a first filter 105_1 through a last filter is provided.The first filter 105_1 through the last filter 105_N connect to a firstinput through a last input (e.g., transmission line 30 individuallyconnected to respective ports 110), respectively. The first filter 105_1through the last filter 105_N each has a first passband through a lastpassband (respectively including frequencies f₁-f_(N)), respectively,such that the first passband through the last passband are eachdifferent.

At block 610, a common output (e.g., transmission line 30 connected tocommon port 120) connected to the first input through the last input viathe first filter 105_1 through the last filter 105_1.

The first input through the last input are each isolated from oneanother, thereby avoiding signal leakage among the first input throughthe last input. The first filter through the last filter are eachconfigured to transmit signals (e.g., microwave signals 150_1-150_N) ata different set of frequencies. The first filter through the last filter(e.g., bandpass filters 105_1-105_N) are each passive thereby requiringno operational power to operate as passive filters, and requiring nopower gain.

The first filter 105_1 of the first filter through the last filter isconfigured to only pass the signals at a first set of frequencies, thenext filter 105_2, of the first filter through the last filter isconfigured to only pass the signals at a next set of frequencies, andthe last filter 105_N of the first filter through the last filter isconfigured to only pass the signals at a last set of frequencies. Eachof the first set, next set, and last set of frequencies arenon-overlapping (i.e., the passbands do not overlap).

The first input through the last input include a first port (e.g., port1) through a last port (e.g., port N), respectively. The first port 1through the last port N are operatively connected to the first filter105_1 through the last filter 105_N, respectively, such that a firstthrough last signals (e.g., microwave signals 150_1-150_N or microwavesignals 150_1-150_Z in FIG. 4) respectively input through the first portthrough the last port are to be combined and output through a commonport 120.

A first impedance transformer 505_1 through a last impedance transformer505_N are respectively connected in between the first port 1 through thelast port N and the first filter 105_1 through the last filter 105_N.The first impedance transformer through the last impedance transformerare configured to provide impedance matching as discussed in FIG. 5. Acommon impedance transformer 510 is connected between the first filter105_1 through the last filter 105_N and the common port 120, and thecommon impedance transformer 510 is configured to provide impedancematching. The first filter through the last filter are superconducting,and the first filter through the last filter including superconductingmaterials.

FIG. 7 is a flow chart of a method 700 of configuring a superconductingmicrowave distributer device 100 according to one or more embodiments.Reference can be made to FIGS. 1-6. The superconducting microwavedistributer and the superconducting microwave combiner are the samedevice. However, the microwave distributer and combiner operate inopposite directions as discussed. Particularly, the input ports andoutput ports are utilized in reverse order with respect to input andoutput microwave signals 150.

At block 705, a first filter through a last filter is provided. Thefirst filter 105_1 through the last filter 105_N connect to a firstoutput through a last output (e.g., transmission line 30 individuallyconnected to respective ports 110), respectively. The first filter 105_1through the last filter 105_N has a first passband through a lastpassband (respectively including frequencies f₁-f_(N)), respectively,such that the first passband through the last passband are eachdifferent.

At block 710, a common input (e.g., transmission line 30 connected tocommon port 120) is connected to the first output through the lastoutput via the first filter 105_1 through the last filter 105_N.

The first output through the last output are each isolated from oneanother, thereby avoiding signal leakage among the first output throughthe last output. The first filter through the last filter are eachconfigured to transmit signals (e.g., microwave signals 150_1-150_N) ata different set of frequencies. The first filter through the last filter(e.g., bandpass filters 105_1-105_N) are each passive thereby requiringno operational power to operate as passive filters, and generating nopower gain.

The first filter 105_1 of the first filter through the last filter isconfigured to only pass the signals at a first set of frequencies, thenext filter 105_2, of the first filter through the last filter isconfigured to only pass the signals at a next set of frequencies, andthe last filter 105_N of the first filter through the last filter isconfigured to only pass the signals at a last set of frequencies. Eachof the first set, next set, and last set of frequencies arenon-overlapping (i.e., the passbands do not overlap).

The first output through the last output include a first port (e.g.,port 1) through a last port (e.g., port N), respectively. The first port1 through the last port N are operatively connected to the first filter105_1 through the last filter 105_N, respectively, such that a firstthrough last signals (e.g., microwave signals 150_1-150_N or microwavesignals 150_1-150_Z in FIG. 4) respectively output through the firstport through the last port. The first through last signals (e.g.,microwave signals 150_1-150_N or microwave signals 150_1-150_Z in FIG.4) are together input through a common port 120 at differentfrequencies.

A first impedance transformer 505_1 through a last impedance transformer505_N are respectively connected in between the first port 1 through thelast port N and the first filter 105_1 through the last filter 105_N.The first impedance transformer through the last impedance transformerare configured to provide impedance matching as discussed in FIG. 5. Acommon impedance transformer 510 is connected between the first filter105_1 through the last filter 105_N and the common port 120, and thecommon impedance transformer 510 is configured to provide widebandimpedance matching. The first filter through the last filter aresuperconducting, and the first filter through the last filter includesuperconducting materials.

One or more embodiments include a superconducting system 300. A firstfilter 105_1 through the last filter 105_N is configured to connect tothe first quantum system 305_1 through the last quantum system 305_N,respectively. The first filter through the last filter has a firstpassband through a last passband (respectively including frequenciesf₁-f_(N)), respectively, such that the first passband through the lastpassband are each different. A common output (e.g., transmission line 30connected to common port 120) is connected to the first quantum system305_1 through the last quantum system 305_N via the first filter 105_1through the last filter 105_N.

The first quantum system 305_1 through the last quantum system 305_N areconfigured to resonate at a first resonance frequency (e.g., frequencyf₁) through a last resonance frequency (f_(N)). The first filter throughthe last filter are configured to operate in transmission (pass/transmitthe signal) for the first resonance frequency through the last resonancefrequency respectively, such that each of the first filter through thelast filter is associated with (only) one of the first resonancefrequency f₁ through the last resonance frequency f_(N).

The first filter through the last filter are configured to operate inreflection (i.e., to block) for any other ones of the first resonancefrequency through the last resonance frequency except the associated oneof the first resonance frequency through the last resonance frequency.In other words, the frequencies f₁-f_(N) are selected to match/overlapits own one of the quantum system 305_1-305_N resonance frequency on aone-to-one basis.

Technical effects and benefits include techniques and devices whichseparate the microwave signals based on their frequency thereby allowingthe device to multiplex the readout and drive of multiple qubits withoutattenuating the microwave signals used in the drive and/or readout.Technical benefits further include isolation between the different portsin a power combiner and signal distributer.

The term “about” and variations thereof are intended to include thedegree of error associated with measurement of the particular quantitybased upon the equipment available at the time of filing theapplication. For example, “about” can include a range of ±8% or 5%, or2% of a given value.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams can represent a module, segment, or portionof instructions, which includes one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block can occur out of theorder noted in the figures. For example, two blocks shown in successioncan, in fact, be executed substantially concurrently, or the blocks cansometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments described herein. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdescribed herein.

What is claimed is:
 1. A method for operating a superconductor combiner,the method comprising: receiving microwave signals entering at portscoupled to one or more qubits; combining the microwave signals via acommon node such that respective ones of the microwave signals areprevented from returning back into any of the ports from which therespective ones did not enter; and outputting a combination of themicrowave signals at a common port.
 2. The method of claim 1, whereinthe ports are configured to receive input of the microwave signals.
 3. Amethod for operating a superconductor combiner, the method comprising:receiving microwave signals entering at ports; combining the microwavesignals via a common node such that respective ones of the microwavesignals are prevented from returning back into any of the ports fromwhich the respective ones did not enter; and outputting a combination ofthe microwave signals at a common port, wherein the microwave signalscorrespond to quantum signals from quantum systems comprising qubits. 4.The method of claim 1, wherein the superconductor combiner comprises theports and the common port.
 5. The method of claim 1, wherein quantumsystems are respectively coupled to the ports.
 6. The method of claim 1,wherein the combination of the microwave signals are output to anamplifier.
 7. The method of claim 1, wherein an amplifier is coupled tothe common port.
 8. The method of claim 6, wherein the amplifiercomprises a frequency range operable to amplify the microwave signals.9. The method of claim 1, wherein the microwave signals have differentfrequencies.
 10. The method of claim 1, wherein each of the ports isassociated with a predefined passband.
 11. The method of claim 1,wherein filters are coupled to the ports.
 12. The method of claim 11,wherein the filters are each passive.
 13. The method of claim 11,wherein each of the filters is operable to communicate with a quantumsystem.
 14. The method of claim 1, wherein filters isolate the ports,thereby avoiding signal leakage between the ports.
 15. A method foroperating a superconductor distributer, the method comprising: receivinga combination of microwave signals entering at a common port;individually distributing the microwave signals via a common node; andoutputting the microwave signals at ports coupled to one or more qubitssuch that respective ones of the microwave signals are prevented fromexiting any of the ports for which the respective ones are notdesignated to exit.
 16. The method of claim 15, wherein the microwavesignals have different frequencies.
 17. The method of claim 15, whereineach of the ports is associated with a predefined passband.
 18. Themethod of claim 15, wherein filters are coupled to the ports.
 19. Themethod of claim 18, wherein the filters are each passive.
 20. The methodof claim 15, wherein filters isolate the ports, thereby avoiding signalleakage between the ports.